Historically, for many applications requiring high chemical and temperature resistance, asbestos-containing gaskets have been used in the automotive, marine, chemical, machine and other industries. Due to current health and environmental concerns relating to asbestos, its use has been significantly reduced and other materials sought for these applications.
One such material is expanded-graphite, also known as graphite foil or flexible graphite. Expanded-graphite sheet is made by treating natural graphite with an acid mixture such as concentrated sulfuric acid and nitric acid or concentrated sulfuric acid and hydrogen peroxide and then, by rapidly heating the treated graphite to the decomposition temperature of the acid mixture the graphite expands up to 200 times its initial thickness. After the graphite has been expanded it can be flattened into a uniform sheet by pressing, rolling, or other such process.
Expanded-graphite sheet has many properties that make it desirable as a gasket material. It has excellent chemical resistance and can be used in almost any fluid, with the exception of strongly oxidizing fluids. It can be used over a very broad temperature range; -270.degree. C. to 3650.degree. C. in an oxygen-free atmosphere or up to 600.degree. C. in air. It has low creep relaxation, low permeability to liquids and gases, and has good resilience, conformability and flexibility properties. Also, it is anisotropic with respect to thermal conductivity, thermal expansion, and electrical conductivity. However, it has a serious drawback in that it is a weak and fragile material and must be frequently used in a composite form with a metal support structure.
Polytetrafluoroethylene (PTFE) and other fluoropolymers also have many good mechanical and chemical properties that make them desirable as a gasket material. Polytetrafluoroethylene is tough and flexible even at extremely low temperatures. It has a broad use-temperature range; from -200.degree. C. to 260.degree. C. It has excellent resistance to chemicals and is stable to all acids, alkalis, and organic chemicals. It is only slightly attacked by molten alkali metals and by fluorine, chlorine trifluoride, etc., at high temperatures. It has excellent frictional properties, particularly at low speeds and with high loads, having coefficient of friction values in the range 0.04 to 0.06. It also has exceptional non-stick or release properties so that it does not readily adhere to other materials. The non-stick properties are advantageous to its use as a gasket but are a serious drawback when its attachment to another material is desired. Further drawbacks associated with fluoropolymers are that they exhibit high creep relaxation and have almost no resilience.